Apparatus for controlling the intensity of vehicle headlamps

ABSTRACT

Apparatus for controlling the intensity of light from the dip filaments of vehicle headlights in accordance with the intensity of street lighting is described. Signals from a light sensor on the vehicle are processed to remove any constant intensity component due to daylight for example so that the resultant signal depends only on a cyclically variable component due to the cyclic variation in the intensity of street lighting supplied from an a.c. supply. When a vehicle passes between street lamps the amplitude of the said variable component varies, and for this reason means are provided to control the electrical supply to the dip filaments so that the intensity of light from these filaments is inversely proportional to minima in the amplitude of the variable component. In the embodiments specifically described the supply is disconnected from the filaments for fixed periods, the repetition frequency of the periods being varied to satisfy the above mentioned condition.

United States Patent 91 Moore et al.

[ 51 Aug. 6, 1974 APPARATUS FOR CONTROLLING THE INTENSITY OF VEHICLE I-IEADLAMPS Primary ExaminerHerman Karl Saalbach Assistant Examiner.lames B. Mulling [75] Inventors. Ronald Leslie Moore, Ascot, Dennis Ammgy, Agent, or firm cushman, Darby &

Geoffrey Wallace Mace, Camberlcy, I

Cushman both of England [73] Assignee: The Secretary of State for the Environment in Her Britannic 57 ABSTRACT Majesty s Government of the United Kingdom of Great Britain and Apparatus for controlling the intensity of light from Northern Ireland, London, England the dip filaments of vehicle headlights in accordance with the intensity of street lighting is described. Sig- [221 Filed 1972 nals from a light sensor on the vehicle are processed [21] Appl. No.2 307,833 to remove any constant intensity component due to daylight for example so that the resultant signal dea Apphcamn Data pends only on a cyclically variable component due to [63] commuat'on'm'pan of 87,105 the cyclic variation in the intensity of street lighting abandoned supplied from an ac. supply. When a vehicle passes between street lamps the amplitudeof the said vari- [301 Forelgn Apphcatlon Pnomy Data able component varies, and for this reason means are NOV. 6, i969 Great Britain 54521/69 provided to control the electrical upply to the filaments so that the intensity of light from these fila- U-S- ments is inversely proportional to minima in the am i 159 plitude of the variable component. In the embodi- [5 hit. ments pecifically described the upply is disco Fleld 0 Search 3 83, nected from the filaments for fixed periods the -epeti- 315/156-159 tion frequency of the periods being varied to satisfy the above mentioned condition. [56] References Cited UNITED STATES PATENTS 12 Claims, 13 Drawing Figures 3.214.640 10/1965 Mills 3l5/l59 X V ,/0 ,m 5 LIGHT a A.C. D.C. PEAK DETECTOR AMP. RESTORATION DETECTOR 1 LOW LEVEL PULSE DETECTOR GENERATOR t /9 I INVERTOR DlGlTAL TO COUNTER ANALOG COM PAEATOR CONVERTER RESET HEAD i LAMP SWITCHING l 1 HEAD 20 2i i LAMP BATTERY PATENTEU G 6 974 SHEET 3 [IF 4 TIME TIME

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APPARATUS FOR CONTROLLING THE INTENSITY OF VEHICLE HEADLAMPS This application is a continuation-in-part of our copending application Ser. No. 87,105, filed Nov. 5, 1970, now abandoned.

The present invention relates to controlling vehicle lights in accordance with the intensity of street lighting, and particularly to dimming dipped headlamps.

Driving at night would be simplified if headlamps could be switched on and left on. However with headlamps on, it is found that in good street lighting there may be some difficulty in seeing clearly because the glare caused by other vehicles headlamps more than offsets any possible improvement in vision due to the headlamps of ones own vehicle.

To overcome this a semi-automatic dimmed headlamp system has been invented and is disclosed in U.K. paten specifications Nos. 1,074,627 and 1,111,157. In this system the headlamps are automatically switched on at a reduced intensity of, for example l/5 to I th normal intensity, when both the ignition key is turned on and the sidelamps are switched on. This semiautomatic system still requires the driver to remember to revert to the ordinary dipped beam on entering a poorly lit street or unlit area and to this extent it is not foolproof.

It is a primary object of the invention to provide apparatus for controlling vehicle lamps in dependence upon the intensity of electrical street lighting supplied from an alternating current source, comprising conversion means including light-sensitive means, the conversion means being adapted to provide an output signal dependent on the amplitude of intensity variations at twice the frequency of the alternating current source in light incident on the vehicle from sources remote from the vehicle but independent of any substantially constant intensity component in said incident light, and control means for controlling a lamp in dependence upon a characteristic of the said output signal of the conversion means.

The intensity of light from the lamp may be controlled, and/or the lamp may be switched on or off, and/or switching between different filaments may be controlled.

When the apparatus is used to control dipped headlamps it must be such that the intensity of light from the headlamps varies inversely with the general level of street lighting incident on the light-sensitive means.

Apparatus according to the invention has the advantage that it is able to measure the quality of street lighting and adjust dipped headlamp intensity accordingly, and at the same time distinguish between street lighting and daylight by responding only to variations in light intensity at twice the frequency of the electrical supply for the street lighting. This is essential to enable the full power of the headlamp dipped beams to be used in daylight fog or other conditions of poor daylight visibility.

As a vehicle driving along a lit street passes from one street lamp to the next, it passes through relatively bright and dark areas. As a result the amplitude of variations in light incident on the vehicle at the supply frequency have maxima and minima which reflect the quality of street lighting. Preferably, therefore, headlamp light intensity varies inversely with the minimum amplitude of variations in incident light intensity.

The conversion means may include an a. c. amplifier to remove any d.c. component in the signal from the light sensitive means due to constant intensity incident light.

The control means may include a dc. restoration circuit coupled to the ac. amplifier, the output of the dc. restoration circuit being coupled to a peak-detector circuit, by way of a low-level detector circuit for detecting the minima in the output signal from the peakdetector circuit.

The low-level detector circuit may comprise a series resistor and a shunt capacitor connected between a pair of input and a pair of output terminals with the capacitor adjacent the output terminals, a differential amplifier having input terminals connected to the input and output terminals, respectively, and switch means, preferably a transistor, connected across the output terminals, the amplifier being connected to discharge the capacitor when the voltage across the input terminals falls below that across the output terminals.

The control means may also include means for generating constant duration pulses, a counter for counting the pulses, a comparator for comparing the number reached by the counter with the output of an inverter coupled to the low-level detector circuit, and logic means coupled to the comparator for resetting the counter and switching off a lamp controlled by the apparatus for a fixed interval, every time the number reached by the counter has a predetermined relationship with the output from the low-level detector circuit.

Certain embodiments of the invention will now be described by way of example with reference to the accompanying drawings, in which:-

FIG. 1 is a block' schematic diagram of a first embodiment of apparatus according to the invention,

FIG. 2 is a schematic circuit diagram of a low-level detector circuit used in FIG. 1,

FIG. 3 is a part-circuit part-block diagram of a second embodiment of apparatus according to the inventron,

FIGS. 4(a), (b), (c), (d) and (e) are waveforms (not drawn to scale) used in explaining the operation of the circuit of FIG. 1, and

FIGS. 5(a), (b), (c), (d) and (e) are waveforms (not drawn to scale) used in explaining the operation of the circuit of FIG. 3.

When a vehicle passes along a road lit by electric street lighting, the intensity of light incident on the car, varies at twice the frequency of the power of supply for the street lamps. A further variation is superimposed on the incident light by the vehicle passing street lamps. The incident light may also have a steady component due to twilight at dawn or dusk, and light from filament lamps whose filaments do not entirely cool between power supply cycles. The overall instantaneous waveform is shown at in FIG. 4, the relatively high frequency variations being due to the frequency of the power supply. An envelope 71 is shown with relatively low frequency variations due to the vehicle passing through areas of varying the brightness as street lamps are passed. A constant intensity component due to the lamp filaments not cooling completely and/or twilight is indicated by the line 72 but this component is not always present. A change in the general level of lighting occurs between envelope peaks 73 and 74 as indicated by the difference in heights of these peaks. Hence the incident light has a unidirectional waveform which varies periodically at twice the frequency of the power supply and at a lower frequency due to movement of the vehicle. The minimum of this waveform may or may not reach zero.

In FIG. 1 a light-sensitive detector is mounted on a vehicle in a position where it can receive ambient light. The detector 10 is coupled to an a.c. amplifier 11 whose output when constant intensity light only is received, such as during daylight and in the absence of varying intensity light sources is zero, since it receives a substantially d.c. input from the detector. The a.c. amplifier is such that its response to low frequency variations, such as those of the envelope 72, is low. At night in street lighting, the amplifier will receive an a.c. signal from the detector due to the variations in light intensity and its output will be in the form shown at 75 in FIG. 4(b) with an envelope 76. The a.c. amplifier 11 is coupled to a d.c. restoration circuit 12 which provides an output waveform with the minima of the a.c. signal restored to zero voltage as shown at 77 in FIG. 4(c) with envelope 78. A peak detector circuit 13 which is constructed to detect the peaks of the relatively high frequency is coupled to the d.c. restoration circuit 12 to provide a d.c. signal whose output varies in proportion to the peaks of the d.c. restored signal. In effect the detector circuit 13 provides a signal similar to the envelope 78. A low-level detector circuit 14 coupled to the peak detector circuit 13 provides a d.c. signal which is proportional to the magnitude of the minima in the signal from the peak detector circuit; that is to the minima of the relatively low frequency variation. The circuit 14, which is described in more detail below, responds quickly to reductions in these minima and slowly if they increase. Thus the output from the low-level detector circuit 14 as shown in FIG. 4(a') varies in proportion to the minimum values of the a.c. signal generated by the light-sensitive detector 10.

A further output of the d.c. restoration circuit 12 is coupled to a pulse generator 15 which amplifies the d.c. restored signal and clips it to provide rectangular pulses of constant duration as shown in FIG. 4(e). These pulses are fed to a counter 16 and then by way of a digital-to-analogue converter 17 to a comparator 18. The output of the low-level detector circuit 14 is also passed to the comparator 18 by way of an inverter 19 to provide a reference level which varies inversely with minima in the low frequency light intensity variations. When the output of the inverter equals that of the digital-to-analogue converter, the comparator enables a gate 20, allowing a pulse from pulse generator 15 to pass to a switching circuit 21 which disconnects the dip filaments of the headlamps 22 and 23 of the vehicle from a battery 25 for the duration of that pulse. At the same time the pulse passing through the gate 20 resets the counter 16 by way of a reset circuit 24. The switching circuit 21 may simply be a transistor switch in the circuit supplying the dipped-headlight filaments.

In operation, therefore the dipped-headlight filaments of the headlamps 22 and 23 are switched off for a short interval every time the number counted by the counter 16 reaches a variable total proportional to the inverse of the minimum street lighting intensity. As the minimum street lighting intensity rises or falls the headlamps are switched off more frequently or less frequently, respectively.

In the low-level detector circuit of FIG. 2, the voltage V,-,, whose lowest level is to be remembered appears as a voltage developed across a resistor 26. This voltage is transferred to a resistor 27 by a complementary pair of transistors T and T As a result condenser C charges to an output voltage V,,. Transistors T T T and T form a balanced amplifier with high common mode rejection obtained by the constant bias current transistor pair T and T A resistor 28 determines the constant current since the transistors T and T are chosen to be matched and to have the same base emitter potential and the transistor T is virtually saturated. The output of the balanced amplifier drives a transistor T single ended. This arrangement allows the voltage V',-,, across the resistor 27 to have large excursions without overloading the circuit.

If the voltage V,-,, is less positive than the voltage V,,, the transistor T conducts as does the transistor T-,. As a consequence the condenser C discharges until such time as V, is close to V,-,,. The size of this offset in voltage depends upon the loop gain and the choice of the resistor 28. The transistors T and T now cease conduction. Further reduction of V',,, forces V to follow downwards. If V',,, rises there is no immediate change in V but changes in V',,, will be integrated by the time constant of a resistor 29 and the capacitor C. This time constant is made long enough to suit the memory of lowest level required by the application, that is in this case the frequency in the minima of street lamps passed at, for example, l5 miles per hour.

Hence the output voltage V, indicates when the input level is below the remembered level and falls quickly with the remembered level. Rises in input are followed much more slowly.

The circuit of FIG. 2 has the advantage that a low difference between the voltages V' and V can be detected. Once the very stable offset between the transistors T and T, has been allowed for, this difference is of the order of a few milli-volts.

In a second embodiment of the invention shown in FIG. 3 a photo transistor 31, 5.0.5. Fairchild Type BPY 60, is positioned on a vehicle to receive ambient light. The collector of the photo transistor 31 is con nected to the input of an amplifier 32 which is one of the four amplifiers contained in an RCA integrated circuit type CA 3048. The amplifier 32 is a single ended amplifier with internal d.c. connected negative feedback and in addition the output is connected via a feedback path containing a notch network 33, to the base of the transistor 31. The notch network 33 is a well known type of circuit, often referred to as a bridged T network, having a loss frequency characteristic which is low at all frequencies except those in the immediate region of 100 Hz. Such networks are described at page 221 et. seq. in Basic Feedback Control System Design by C. S. Savant, published'by McGraw Hill in 1958. Thus the amplifier 32 has less negative feedback at around 100 Hz and therefore a high gain at this frequency. This arrangement also provides a high degree of stabilisation of the d.c. working point of the amplifying system, since there is high d.c. feedback.

The output of the amplifier 32 is proportional to any 100 Hz component, such as that shown at in FIG. 5(a), in light falling on the photo transistor 31, this being the frequency of intensity variations in light from street lighting supplied from a 50 Hz supply. Where the street lighting is supplied from a 60 Hz supply the amplifier 32 is constructed to have high gain at around 120 Hz. The envelope of the incident light waveform is designated 81. Since the amplifier 32 is, in effect, sensitive to the street lighting supply frequency, its output is independent of any constant intensity component 82 in light incident on the photo transistor 31.

A detector circuit 35 which is one of two contained in a Texas Instrument Ser. No. 7,525N sense amplifier package is coupled to the output of the amplifier 32. The detector 35 has two possible output levels, one which represents a binary l and one which represents a binary 0. The level representing the binary l is generated as long as the instantaneous value of the input signal exceeds a reference voltage indicated at 87 in FIG. 5(b) showing the output waveform of the amplifier 32. Thus the output of the detector 35 is a square wave with a mark/space ratio which is determined by the amplitude of the input signal. This detector output waveform is shown in FIG. 5(0) where it will be seen that the mark/space ratio is high in regions 83 and 84 corresponding to peaks in incident light 83' and 84' where two street lamps are comparatively bright. In regions on either side of these lamps, the lighting is not so good, and peaks 85 and 86' occur with the corresponding mark/space ratios 85 and 86. Between the street lamps the waveform dips and the mark/space ratio falls.

An inversion is carried out by the transistor 36 so that the logic level representing a binary l is present at its collector when the input to the amplifier 35 is below the reference voltage. The transistor 36 is coupled to cause a transistor 37 to discharge a capacitor 38 when the transistor 37 conducts but at other times the capacitor 38 is charged from a terminal 34 connected to a reference voltage source (not shown). Hence the capacitor 38 is charged for a time equal to the spaces in the square wave output of the detector 35 and discharged during the marks of this square waveform. The waveform across the capacitor 38, shown at 88 in FIG. 5(d), therefore has peaks which are inversely proportional to the intensity of light falling on the photo transistor 31.

A capacitor 39 and a differential amplifier 41 together form a peak detector circuit which provides a voltage across the capacitor 39 proportional to peak values of the voltage across the capacitor 38. The capacitor 39 charges through a transistor 42 until its voltage equals that across the capacitor 38 when the differential amplifier 41 switches off the transistor 42. The capacitor 39 then discharges slowly through a resistor 43 until a new peak voltage appears on the capacitor 38. A resistor 44 in series with the capacitor 39 and the transistor 42 is provided to limit the rate of charge of the capacitor 39 to prevent isolated low value cycles of the 100 Hz signal from causing a high voltage to be developed across the capacitor 39. Choice of the resistor 43 governs the time constant for discharging the capacitor 39 and regulates the rate at which increases in the amplitude of the 100 Hz signal are followed.

The voltage across the capacitor 39 therefore varies inversely with the overall level of street lighting as shown in H6. 5(e) where it will be seen that the change to high level lighting is followed slowly at 89, while the change to low level lighting is followed comparatively quickly at 90. The intensity of the dipped headlight beam has to be controlled in accordance with the voltage across the capacitor 39, and the required variation of the headlight intensity is achieved by switching off the dip filaments of the headlamps for short fixed duration intervals. Control of intensity is effected by adjusting the repetition frequency of the intervals.

A capacitor 45 is allowed to charge until its voltage equals that across the capacitor 39 when a differential amplifier 46 triggers a monostable circuit 47. The dip filaments of headlights 48 and 49 are coupled to the monostable circuit 47 to be switched off for its quasistable period. Hence the voltage across the capacitor 39 acts as a variable reference level, controlling the frequency at which the monostable circuit 47 enters its quasi-stable state.

The differential amplifier 46 is contained in the RCA. package CA3051 together with the amplifier 41; and the monostable circuit 47 is a Texas lnstruments Ser. No. 7412lN monostable multi-vibrator.

In order to achieve approximately :1 variations in the time for which the headlamps are on, the charging of the capacitor 45 is not carried out by a simple resistor. At first the capacitor 45 charges quickly to a voltage determined by the ratio of the resistances of resis tors 51 and 52. The capacitor 45 then continues to charge at the same time as a capacitor 53 and the rate of growth of voltage across the capacitor 45 is governed by the rate of growth of voltage across the capac itor 53. The initial growth of voltage across the capacitor 45 depends on the time constant of the resistor 51 and the capacitor 45, and later the rate of growth depends on the time constant of the resistors 51 and 52 and the capacitor 53.

When the differential amplifier 46 passes an output pulse to the monostable circuit 47, the field effect transistors 54 and 55 are turned on by way of a capacitor 61 and the capacitors 45 and 53 are discharged. The discharged condition is held until the monostable circuit regains its stable condition.

The monostable circuit 47 is coupled by way of a NAND buffer circuit 56 (Texas Instruments Ser. No. 7440) to a grounded emitter transistor 57 which itself buffers the stage 56. The transistor 57 drives a high power S.G.S. Fairchild type CP701 transistor 58 which when turned on switches off two further transistors 59 and 60 of the same type. Since the transistors 59 and 60 are in series with the dip filaments of the headlights 48 and 49, when the monostable circuit 47 is in its quasi-stable state, the dip filaments of the headlights 48 and 49 are off.

in the above description components shown in FIG. 3 and used for biasing, coupling, or decoupling have not been specifically mentioned since their functions are well known.

Although two specific circuits for carrying out the invention have been described, it will be clear that many other circuits could be used.

Furthermore the invention may be applied to the main-beam filaments of vehicle headlamps, and to control the switching on and off of the dip and/or mainbeam filaments. For example when the variation due to street lighting rises above a predetermined level, circuits controlling the headlamps may be triggered to switch on the headlamps or the dip or main-beam filaments thereof. When this variation is no longer sensed the headlamps are switched off if daylight is sensed, or the dip filaments are switched off and main-beam filaments are switched on if ambient light is below a predetermined level.

We claim:

1. Apparatus for controlling vehicle lamps in dependence upon the intensity of electrical street lighting supplied from an alternating current source, comprising conversion means including light-sensitive means, the conversion means being adapted to provide an output signal dependent on the amplitude of intensity variations at twice the frequency of the alternating current source in light incident on the vehicle from sources remote from the vehicle but independent of any substantially constant intensity component in said incident light, and control means for controlling a vehicle lamp in dependence upon a characteristic of the said output signal of theconversion means.

2. Apparatus according to claim 1 wherein the control means is adapted to control the intensity of light from a lamp.

3. Apparatus according'to claim 1 wherein the light sensitive means provides a unidirectional signal dependent upon the intensity of light incident on the vehicle, and the conversion means includes an alternating current amplifier, coupled to receive the unidirectional signal, for providing the said output signal of the conversion means in the form of an alternating current signal, the amplitude of which is proportional to the amplitude of the said intensity variations.

4. Apparatus according to claim 3 wherein the said characteristic is the magnitude of amplitude minima in the alternating current signal, and the magnitude of amplitude maxima in the alternating current signal do not substantially affect the control of the vehicle lamp, and the control means includes means for providing a control signal dependent on the magnitude of the said minima but not substantially on that of said maxima, and means for varying the intensity of light from the lamp inversely in proportion to the magnitudes of the said minima.

5. Apparatus according to claim 4 wherein the control means includes a dc. restoration circuit coupled to the output of the alternating current amplifier, the dc. restoration circuit being coupled by way of a peakdetector circuit to a low-level detector circuit for detecting minima in the output signal from the peakdetector circuit.

6. Apparatus according to claim 3 wherein the alternating current amplifier has a high gain at the frequency of the said intensity variations compared with its gain at other frequencies.

7. Apparatus according to claim 3 wherein the said characteristic is the magnitude of amplitude minima in the alternating current signal and the control means includes detector means for providing a square-wave signal, the mark/space ratio of which depends on the amplitude of the alternating current signal, the detector means being connected to charge and discharge a capacitor alternately, the durations of charge and discharge intervals being dependent on the said mark/space ratio, and peak detector means for detecting peaks in the voltage across the capacitor, the apparatus being such that the magnitudes of the said peaks are proportional to minima in the said intensity variations.

8. Apparatus according to claim 3 wherein the control means controls the intensity of light from the dip filaments of the headlights of a vehicle.

9. Apparatus for controlling vehicle lamps in dependence upon the intensity of electrical street lighting supplied from an alternating current source, comprismg conversion means including light-sensitive means,

the conversion means being constructed to provide an output signal dependent on the amplitude of intensity variations at twice the frequency of the alternating current source in light incident on the vehicle from sources remote from the vehicle but independent of any substantially constant intensity component in said incident light, and

control means for controlling a vehicle lamp in dependence upon a characteristic of the said output signal of the conversion means,

wherein the light sensitive means provides a unidirectional signal dependent upon the intensity of light incident on the vehicle, and the conversion means includes an alternating current amplifier, coupled to receive the unidirectional signal, for providing the said output signal of the conversion means in the form of an alternating current signal, the amplitude of which is proportional to the amplitude of the said intensity variations,

wherein the said characteristic is the magnitude of amplitude minima in the alternating current signal and the control means includes means for providing a control signal dependent on the magnitude of the said minima, and means for varying the intensity of light from the lamp inversely in proportion to the magnitudes of the said minima,

wherein the control means includes a dc. restoration circuit coupled to the output of the alternating current amplifier, the dc. restoration circuit being coupled by way of a peak-detector circuit to a lowlevel detector circuit for detecting minima in the output signal from the peak-detector, and wherein the low-level detector circuit comprises a series resistor and a shunt capacitor connected between a pair of input and a pair of output terminals with the capacitor adjacent the output terminals, a differential amplifier having input terminals connected to the input and output terminals, respectively, and switch means connected across the output terminals, the amplifier being connected to discharge the capacitor when the voltage across the input terminals falls below that across the output terminals.

10. Apparatus according to claim 9 wherein the control means includes means for generating constant duration pulses, a counter for counting the pulses, a comparator for comparing the number reached by the counter with the output of an inverter coupled to the low-level detector circuit, and logic means coupled to the comparator for resetting the counter and switching off a lamp controlled by the apparatus for a fixed interval, every time the number reached by the counter has a predetermined relationship with the output from the low-level detector circuit.

1 1. Apparatus for controlling vehicle lamps in depen dence upon the intensity of electrical street lighting supplied from an alternating current source, comprismg conversion means including light-sensitive means,

the conversion means being constructed to provide an output signal dependent on the amplitude of intensity variations at twice the frequency of the altemating current source in light incident on the vehicle from sources remote from the vehicle but inwherein dependent of any substantially constant intensity component in said incident light, and

control means for controlling a vehicle lamp in dependence upon a characteristic of the said output signal of the conversion means,

wherein the light sensitive means provides a unidirectional signal dependent upon the intensity of light incident on the vehicle, and the conversion means includes an alternating current amplifier, coupled to receive the unidirectional signal, for providing the said output signal of the conversion means in the form of an alternating current signal, the amplitude of which is proportional to the amplitude of the said intensity variations,

wherein the said characteristic is themagnitude of amplitude minima in the alternating current signal and the control means includes detector means for providing a square-wave signal, the mark/space ratio of which depends on the amplitude of the alternating current signal, the detector means being connected to charge and discharge a capacitor alternately, the durations of charge and discharge intervals being dependent on the said mark/space ratio, and peak detector means for detecting peaks in the voltage across the capacitor, the apparatus being such that the magnitudes of the said peaks are proportional to minima in the said intensity variations, and

the control means includes voltagecomparison means for comparing a rising voltage with the output voltage from the peak detector circuit, and a monostable circuit coupled to the voltagecomparison means to enter its quasi-stable state each time the rising voltage equals the said output voltage, the monostable circuit being coupled to switch off the said lamp when in the quasistable state, and the rising voltage commencing to rise from a predetermined value each time the monostable circuit enters its stable state.

12. Apparatus for controlling the intensity of vehicle lamps in dependence upon the intensity of electrical 10 street lighting supplied from an alternating current source, comprising conversion means including light sensitive means, the conversion means providing an electrical output signal dependent on the amplitude of intensity variations at twice the frequency of the alternating current source in light incident on the vehicle from said alternating current source in lightincident on the vehicle from said alternating current supplied lighting remote from the vehicle, said output signal from said conversion means being independent of any constant intensity component in the said incident light, and

control means for controlling a lamp, including means for providing a control signal inversely dependent upon the magnitude of minima in the amplitude of the said alternating current output signal from the conversion means but substantially independent of the magnitude of amplitude maxima in the said alternating current output signal and means for rendering the intensity of the vehicle lamp proportional to the said control signal. 

1. Apparatus for controlling vehicle lamps in dependence upon the intensity of electrical street lighting supplied from an alternating current source, comprising conversion means including light-sensitive means, the conversion means being adapted to provide an output signal dependent on the amplitude of intensity variations at twice the frequency of the alternating current source in light incident on the vehicle from sources remote from the vehicle but independent of any substantially constant intensity component in said incident light, and control means for controlling a vehicle lamp in dependence upon a characteristic of the said output signal of the conversion means.
 2. Apparatus according to claim 1 wherein the control means is adapted to control the intensity of light from a lamp.
 3. Apparatus according to claim 1 wherein the light sensitive means provides a unidirectional signal dependent upon the intensity of light inCident on the vehicle, and the conversion means includes an alternating current amplifier, coupled to receive the unidirectional signal, for providing the said output signal of the conversion means in the form of an alternating current signal, the amplitude of which is proportional to the amplitude of the said intensity variations.
 4. Apparatus according to claim 3 wherein the said characteristic is the magnitude of amplitude minima in the alternating current signal, and the magnitude of amplitude maxima in the alternating current signal do not substantially affect the control of the vehicle lamp, and the control means includes means for providing a control signal dependent on the magnitude of the said minima but not substantially on that of said maxima, and means for varying the intensity of light from the lamp inversely in proportion to the magnitudes of the said minima.
 5. Apparatus according to claim 4 wherein the control means includes a d.c. restoration circuit coupled to the output of the alternating current amplifier, the d.c. restoration circuit being coupled by way of a peak-detector circuit to a low-level detector circuit for detecting minima in the output signal from the peak-detector circuit.
 6. Apparatus according to claim 3 wherein the alternating current amplifier has a high gain at the frequency of the said intensity variations compared with its gain at other frequencies.
 7. Apparatus according to claim 3 wherein the said characteristic is the magnitude of amplitude minima in the alternating current signal and the control means includes detector means for providing a square-wave signal, the mark/space ratio of which depends on the amplitude of the alternating current signal, the detector means being connected to charge and discharge a capacitor alternately, the durations of charge and discharge intervals being dependent on the said mark/space ratio, and peak detector means for detecting peaks in the voltage across the capacitor, the apparatus being such that the magnitudes of the said peaks are proportional to minima in the said intensity variations.
 8. Apparatus according to claim 3 wherein the control means controls the intensity of light from the dip filaments of the headlights of a vehicle.
 9. Apparatus for controlling vehicle lamps in dependence upon the intensity of electrical street lighting supplied from an alternating current source, comprising conversion means including light-sensitive means, the conversion means being constructed to provide an output signal dependent on the amplitude of intensity variations at twice the frequency of the alternating current source in light incident on the vehicle from sources remote from the vehicle but independent of any substantially constant intensity component in said incident light, and control means for controlling a vehicle lamp in dependence upon a characteristic of the said output signal of the conversion means, wherein the light sensitive means provides a unidirectional signal dependent upon the intensity of light incident on the vehicle, and the conversion means includes an alternating current amplifier, coupled to receive the unidirectional signal, for providing the said output signal of the conversion means in the form of an alternating current signal, the amplitude of which is proportional to the amplitude of the said intensity variations, wherein the said characteristic is the magnitude of amplitude minima in the alternating current signal and the control means includes means for providing a control signal dependent on the magnitude of the said minima, and means for varying the intensity of light from the lamp inversely in proportion to the magnitudes of the said minima, wherein the control means includes a d.c. restoration circuit coupled to the output of the alternating current amplifier, the d.c. restoration circuit being coupled by way of a peak-detector circuit to a low-level detector circuit for detecting minima in the output signal from thE peak-detector, and wherein the low-level detector circuit comprises a series resistor and a shunt capacitor connected between a pair of input and a pair of output terminals with the capacitor adjacent the output terminals, a differential amplifier having input terminals connected to the input and output terminals, respectively, and switch means connected across the output terminals, the amplifier being connected to discharge the capacitor when the voltage across the input terminals falls below that across the output terminals.
 10. Apparatus according to claim 9 wherein the control means includes means for generating constant duration pulses, a counter for counting the pulses, a comparator for comparing the number reached by the counter with the output of an inverter coupled to the low-level detector circuit, and logic means coupled to the comparator for resetting the counter and switching off a lamp controlled by the apparatus for a fixed interval, every time the number reached by the counter has a predetermined relationship with the output from the low-level detector circuit.
 11. Apparatus for controlling vehicle lamps in dependence upon the intensity of electrical street lighting supplied from an alternating current source, comprising conversion means including light-sensitive means, the conversion means being constructed to provide an output signal dependent on the amplitude of intensity variations at twice the frequency of the alternating current source in light incident on the vehicle from sources remote from the vehicle but independent of any substantially constant intensity component in said incident light, and control means for controlling a vehicle lamp in dependence upon a characteristic of the said output signal of the conversion means, wherein the light sensitive means provides a unidirectional signal dependent upon the intensity of light incident on the vehicle, and the conversion means includes an alternating current amplifier, coupled to receive the unidirectional signal, for providing the said output signal of the conversion means in the form of an alternating current signal, the amplitude of which is proportional to the amplitude of the said intensity variations, wherein the said characteristic is the magnitude of amplitude minima in the alternating current signal and the control means includes detector means for providing a square-wave signal, the mark/space ratio of which depends on the amplitude of the alternating current signal, the detector means being connected to charge and discharge a capacitor alternately, the durations of charge and discharge intervals being dependent on the said mark/space ratio, and peak detector means for detecting peaks in the voltage across the capacitor, the apparatus being such that the magnitudes of the said peaks are proportional to minima in the said intensity variations, and wherein the control means includes voltage-comparison means for comparing a rising voltage with the output voltage from the peak detector circuit, and a monostable circuit coupled to the voltagecomparison means to enter its quasi-stable state each time the rising voltage equals the said output voltage, the monostable circuit being coupled to switch off the said lamp when in the quasi-stable state, and the rising voltage commencing to rise from a predetermined value each time the monostable circuit enters its stable state.
 12. Apparatus for controlling the intensity of vehicle lamps in dependence upon the intensity of electrical street lighting supplied from an alternating current source, comprising conversion means including light sensitive means, the conversion means providing an electrical output signal dependent on the amplitude of intensity variations at twice the frequency of the alternating current source in light incident on the vehicle from said alternating current source in light incident on the vehicle from said alternating current supplied lighting remote from the vehicle, said output signal from said conversion means being independent of any constant intensity component in the said incident light, and control means for controlling a lamp, including means for providing a control signal inversely dependent upon the magnitude of minima in the amplitude of the said alternating current output signal from the conversion means but substantially independent of the magnitude of amplitude maxima in the said alternating current output signal and means for rendering the intensity of the vehicle lamp proportional to the said control signal. 